Liquid samples are often characterized using optical techniques such as photometry, spectrophotometry, fluorometry, or spectrofluorometry. Typically, the liquid is contained in a vessel referred to as a cell or cuvette, two or more of whose sides are of optical quality and permit the passage of those wavelengths needed to characterize the liquid contained therein. When dealing with very small sample volumes (e.g., from about 1 to 2 microliters), it is difficult to create cells or cuvettes small enough to be filled and to permit the industry standard 1 cm optical path to be used. It is also difficult and/or time consuming to clean these cells or cuvettes for use with another sample.
Perspective on the problem of quantifying a 1-2 microliter liquid sample is gained by considering the physical size of such samples. For example, a 1 microliter droplet occupies a cube with 1 mm edge length or a cylindrical volume with height 1 mm and diameter 1.13 mm. In contrast, optical beam dimensions in conventional spectrophotometers are usually much larger. In order to optically quantify samples that have 1-2 microliter volume in a conventional spectrophotometer, the light beam must have a diameter of approximately 1 mm or less.
Absorbance can be measured with path lengths other than 1 cm. The recent advent of small spectrophotometers designed for use with fiber optics has made it possible to consider device geometries not readily possible before.
For example, U.S. Pat. No. 6,628,382, and U.S. Patent Publication 20020140931 disclose an optical instrument in which a narrow beam of light is directed into a microliter sample, by providing light from a broadband light source, via an optical fiber, to a sample stage that consists of a liquid droplet suspended between two multi-mode optical fibers: one source-side fiber and another fiber that guides light to appropriate detection optics, or a “detection-side” fiber. The close proximity between the source-side and detection-side fibers allows enough of the light cone emanating from the source fiber to be collected by the detection-side fiber after passing through a liquid sample.
One drawback with this close coupling approach is that the presence of the fiber ends in the samples can interfere with the insertion of the sample into the sample zone, cleaning of the sample zone, and other access to the sample zone. Another drawback is the inability to change the separation between the ends of the source and detector fibers without significantly altering the amount of light gathered by the detector fiber. Yet another drawback is that the instrument cannot easily be used to measure the transmission of solid samples with thickness greater than a few hundred microns. Further, operation of the instrument depends on the ability to accurately change the height of the sample and therefore the separation between the ends of the source-side and detection-side fibers.
Additionally, in order to allow the introduction of a sample into the sample zone, and to allow access to the sample zone for cleaning and examination, the upper end of the light path is directed through the free end of a pivotable mechanical arm. The light path from the pivot end of the arm to the free end of the arm passes through a rather long, unrestrained, and exposed optical fiber section that loops over from the pivot end of the arm to the free end of the arm, where the end of the fiber must enter the sample vertically in order to properly direct the beam into the sample. This convoluted path increases optical loss (a function of the radius of curvature of bends in the fiber) and imposes constraints on the dimensions of the device, requiring the instrument to have a certain height in order for the optical fiber to approach the sample vertically. Further, the exposed fiber presents the possibility of variable optical transmission as the optical fiber experiences movement as well as risk of breakage. The instrument also may not be easily modified to include additional optical components.